Just a thought, I may be barking up the wrong tree here but you say you fan/pump speed is set to Auto in the bios, could it be that under load the pump is speeding up hence moving the water too fast through the system and not giving it enough time to cool down?
Unfortunately, the pump is not interfacing with the BIOS in any way. The pump runs at a fixed speed, and cannot be altered (its not a d5 or variant). The fans are set to run on full on the BIOS, and are undervolted to 7v to keep the noise levels down.
On a side note, I should have an i3 to play with tomorrow, and my replacement i5 should arrive either tomorrow or on weds, so I can test both of these CPU's in the same rig, to see if the temperatures are similar or not, which should hopefully show whether it is the W/C loop or the old CPU that was the cause of the high load temperatures.
I will update the thread when I have got the system back up and running.